1. Field of the Invention
The present invention relates to an absorber-coupled far-infrared microwave kinetic inductance detector (FIR MKID) array with a symmetric cross pattern which serves dual roles as radiation absorber and superconducting readout. The metal pattern on the inductance detector allows power to be more evenly distributed across the detector area than the prior art, thus, increasing the detector coupling efficiency and sensitivity. Furthermore, the cross absorber pattern allows the output of the detector to readout at more than twice the frequency of a conventional MKID design. Finally, the arrangement of the detector's resonator transmission line minimizes parasitic coupling among the adjacent resonator channels. This arrangement reduces the complexity in resonator calculations and allows more detectors to be frequency multiplexed in a limited microwave frequency readout bandwidth with significantly small crosstalk.
2. Description of the Related Art
A microwave kinetic inductance detector (MKID) is a superconducting photon detector which operates at cryogenic temperatures, typically below 1 degree Kelvin. The MKID is used in high-sensitivity astronomical detection for frequencies ranging from the far-infrared (FIR) to X-rays. The kinetic inductance of the superconducting transmission line forming the MKID is inversely proportional to the density of the Cooper pairs, and thus, the kinetic inductance increases upon photon absorption. When combined with a capacitor, a microwave resonator is formed, in which its resonant frequency changes with the absorption of photons. The resonator-based readout is useful for developing large-format detector arrays, as each kinetic inductance detector can be addressed by a single microwave tone, and many kinetic inductance detectors can be measured using a single broadband microwave channel (i.e., using frequency-divisional multiplexing).
Single-layer uniform co-planar waveguide or parallel coupled transmission lines are commonly used to generate the FIR MKIDs. A microstrip electrical transmission line provides greater confinement and signal control in this detector readout application. Further, planar transmission line structures are less expensive, lighter, and significantly more compact than traditional waveguide technologies.
A conventional microwave kinetic inductance detector (MKID) 10 (see FIG. 1), to which power P is applied, includes a meander or a spiral line resonator pattern 11 of an infrared (IR) absorber 12, disposed on a thin membrane 13 of silicon as a dielectric substrate layer (see FIG. 2 for a cross-sectional view of FIG. 1 along line “A”). The absorber 12 may be made of any superconductor material with a transition temperature lower than niobium, including aluminum, titanium nitride, molybdenum nitride etc.
The microstrip electrical transmission line of the conventional MKID 10 includes a low impedance (Z) conducting strip 14 along two edges thereof, separated from a ground plane 15 by the silicon membrane 13. The conducting strip 14 is a superconductor at FIR frequencies and at microwave frequencies. The microwave component is formed from the microstrip 14 and includes the metal pattern 11 of the FIR absorber 12, which also serves as a superconducting resonator at the readout microwave frequency (i.e., typically between 0.3 and 10 GHz). The parallel-transmission line 11, 12 are at an FIR quarter-wave spacing or distance “d” from a conductor backshort 16.
However, disadvantages of the prior art MKID 10 include: 1) a metal pattern 11 which produces asymmetric power coupling for both horizontal and vertical signals; 2) the uniform transmission line used in the resonator results in a high resonator current confined around the center of the microwave kinetic inductance detector (MKID) 10, thus, limiting the MKID 10 sensitivity, and potentially producing strong crosstalk to the adjacent MKID channels; 3) strong parasitic coupling among resonators which produce uncertainty in the resonance frequency calculation, thus, limiting the minimum frequency spacing among MKID channels for large detector array implementations; and 4) the metal pattern 11 on the membrane 13 results in low microwave operating frequencies, which limits the maximum number of resonator channels that can be readout by a cryogenic amplifier. Accordingly, an FIR MKID 10 that does not have these disadvantages is desired.